Machine tools employ rotary workholding chucks for a variety of operations, including grinding and turning of diameters of parts of rotation. In turning and grinding machines, a chuck body is usually mounted to a rotatable spindle, and the chuck body includes means for grasping a workpiece for rotation. The chuck and workpiece are then rotated in unison to perform a machining operation.
In some instances, it is necessary to machine a second diameter on a workpiece, not concentric with a first machined diameter. The workpiece is then clamped in a rotary chuck or fixture which is adapted for supporting the primary axis off the axis of rotation, so that the part may be rotated about the axis of rotation and subsequently machined, producing the eccentric diameter.
One common workpiece having eccentric diameters, is the crankshaft of an internal combustion engine, where the crankshaft has a main axis of rotation, to be supported in main bearings of an engine, and has integral eccentric crank pins, which are each to be attached to a piston-connecting rod.
Known devices for supporting and driving a crankshaft in a grinding machine, for example, are shown in the following references:
U.S. Pat. No. 4,269,001, of H. G. Bottomley, issued May 26, 1981, entitled Work Clamping Fixture, includes a workrest portion for supporting the main bearing diameter of a double crank pin crankshaft, where a lever action clamp secures the main bearing diameter in the fixture. For a first machining operation, the first crank pin is located coaxially with the rotary axis of the fixture and machine spindle. During such operation, the main bearing diameter and second crank pin diameter gyrate around the first crank pin axis. Then the machine is stopped, the workpiece is unclamped, and the workpiece is rotated about its main bearing axis So that the part is turned 180.degree., and the second crank pin is therefore placed coaxially with the fixture axis of rotation. The part is then reclamped and the second crank pin is machined. The clamping fixture includes a piston powered pivotal clamp finger. The device also includes springs for assisting in movement of the clamp finger, and a rack and pinion device for shifting the finger pivot to provide clearance when loading a workpiece.
U.S. Pat. No. 3,583,108, of K. Oishi et al., issued Jun. 8, 1971, entitled Machine Tool for Grinding Pins of a Crankshaft, is also a fixture for clamping on the main bearing diameter of a crankshaft. An indexing disk is attached to the crankshaft, and several crank pins are alternately positioned onto the central rotary axis of the spindle. Periodically, the workpiece is automatically rotated about its main bearing axis by a pair of gears, one attached to a shaft concentric with the spindle axis, and a mating gear attached to the indexing disk so that the crankshaft can be rolled over. When the respective crank pin is to be positioned about the spindle axis, the indexing disk is rotated to position against a stop finger.
In each of the foregoing patents, the clamp force on the workpiece must be released so that the workpiece can be repositioned relative to the chuck or fixture.
U.S. Pat. No. 5,249,394, of R. E. Griswold, issued Oct. 5, 1993, entitled Chuck Indexing Method, illustrates a crank pin processing machine wherein a chuck body is clamped to a machine spindle during machining operations, and, subsequent to the first operation, the chuck body is relatively rotated with respect to the mounting spindle, to position the crank pins onto the spindle axis. Rotation of the chuck relative to the spindle is accomplished by a gear affixed to the chuck body, driven by a pinion of a hydraulic indexing motor mounted to the machine headstock. After being indexed, the pinion is axially withdrawn from engagement with the gear, and the chuck body is reclamped to the machine spindle for machining the next crank pin diameter.
The aforementioned devices include complex mechanisms for clamping, unclamping, and reclamping either the workpiece or the chuck body when the workpiece is to be indexed about the fixtured axis. And, in all instances, the second diameter to be machined is rotated in the same direction about the spindle axis as when the first diameter was machined.
The present invention obviates disadvantages inherent in the prior art devices, by providing a simplified eccentric index mechanism which is shifted between two positions by inertial forces alone. In it, neither the workpiece nor the chuck body has to be clamped and unclamped for subsequent indexes.